Harvey Anderson, Mozilla's general counsel, and Asa Dotzler, director of Firefox, weighed in with the accusations.

Anderson warned that Microsoft's decision to allow only Internet Explorer 10 in one mode of Windows RT "signals an unwelcome return to the digital dark ages where users and developers didn't have browser choices."

Dotlzer was more direct. "Microsoft is trying to lock out competing browsers when it comes to Windows running on ARM chips," he said.

Their beef stems from Microsoft's decision to deny other browser makers, including Mozilla, access to APIs (application programming interfaces) necessary to run a browser on Windows RT's conventional desktop.

Windows RT - the edition for ARM processors - will offer a Metro mode that features touch-based apps available from the Windows Store. But it also includes a heavily-restricted "desktop" mode that will run only Microsoft code.

Related

Among the software that will run on the Windows RT desktop - Microsoft hasn't given that mode a specific name - will be new versions of Office's Word, Excel, PowerPoint and OneNote; the Windows Explorer file manager; and a "desktop" edition of (IE10).

Off limits to other vendors' code?

"Windows RT supports the Windows desktop experience including File Explorer, Internet Explorer 10 for the desktop, and most other intrinsic Windows desktop features - which have been significantly architected for both touch and minimised power/resource consumption," said Steven Sinofsky, Microsoft's top Windows executive, earlier this year in the company's first detailed description of the new tablet OS.

Sinofsky's comments were interpreted to mean that the RT desktop mode was off limits to other vendors' code.

And that's what raised hackles at Mozilla.

"Windows on ARM [the former name for Windows RT] prohibits any browser except for Internet Explorer from running in the privileged 'Windows Classic' environment," said Anderson. "In practice, this means that only Internet Explorer will be able to perform many of the advanced computing functions vital to modern browsers in terms of speed, stability, and security to which users have grown accustomed."

"On ARM chips, Microsoft gives IE access to special APIs absolutely necessary for building a modern browser that it won't give to other browsers, so there's no way another browser can possibly compete with IE in terms of features or performance," said Dotzler.

He did not spell out those APIs or how Microsoft was limiting access to them, but in a follow-up post on his personal blog, Dotzler elaborated.

"It's not precisely 'running a browser in Classic' that matters for Windows on ARM," Dotzler said. "It's that running a browser in Classic is the only way that Microsoft has allowed us to get access to the APIs that a browser needs to deliver modern capabilities and performance in Classic and Metro. A browser running exclusively in Metro does not have the APIs necessary to compete with IE or any other modern browser."

As Dotzler inferred, there's nothing blocking Mozilla from submitting a Metro version of Firefox to the Windows Store.

Anderson raised the antitrust flag, hinting that Mozilla may reach out to regulators in the US, the European Union, or both.

"Windows on ARM - as currently designed - restricts user choice, reduces competition and chills innovation," Anderson argued. "If Windows on ARM is simply another version of Windows on new hardware, it also runs afoul of the EC browser choice commitments and seems to represent the very behavior the Department of Justice-Microsoft settlement sought to prohibit."

In late 2009, EU regulators struck a deal with Microsoft that requires the company to offer Windows users a "ballot screen" with alternate browsers to install and make the default. The agreement came out of an antitrust complaint filed by Norway's Opera Software, maker of Opera, two years before.

Document on voluntary principles has disappeared

Microsoft is no longer under US government scrutiny in the wake of the landmark antitrust case that also originated with complaints about its browser. The eventual settlement required Microsoft to share its APIs with third-party vendors.

In 2006, Microsoft published a set of voluntary principles it promised to adhere to, including, "Providing opportunities for developers to build innovative products on the Windows platform, including products that directly compete with Microsoft's own products."

Microsoft's general counsel Brad Smith announced the assurances in an address in Washington, DC.

That document, however, has disappeared from Microsoft's corporate citizenship website.

Dotzler noticed. "This is in direct violation of the promises they made to developers, users, and OEMs about browser choice in documents which mysteriously disappeared from Microsoft's site," he said yesterday, and provided a link to a copy.

Both Mozilla and Google have committed to create versions of their current browsers that run on Windows 8 - the edition for Intel and AMD processors - in both the desktop and Metro environments on that version.